The copolymerization of carbon dioxide with epoxides to form poly(alkylene carbonate) polymers was discovered by Inoue and co-workers (Polymer Letters 7, 287(1969)) and described in U.S. Pat. No. 3,585,168. Other processes are described in U.S. Pat. Nos. 3,900,424, 3,953,383 and 3,248,415. Despite the obvious economic advantage associated with the use of an abundant, low cost material like carbon dioxide, the introduction of commercial products that utilize this chemistry has been very slow. One reason has been the practical difficulty in handling the organometallic catalysts required on a commercial scale.
The Inoue catalyst system was prepared by the reaction of diethylzinc with materials containing active hydrogen compounds., e.g., water, dicarboxylic acids, or dihydric phenols. Typical catalyst productivities ranged from 2.0 to 10.0 grams of polymer per gram of catalyst used, with more results falling at the low end of the range. Long polymerization times--24 to 48 hours--were required in order to achieve satisfactory yields and product molecular weight. Some of the phenol-containing catalysts form highly colored material on exposure to air. This can lead to discolored products because of the difficulty in removing all traces of color bodies from the polymer. A high ratio of catalyst to polymer was required to achieve satisfactory yields; therefore, separation of the catalyst from the polymer at the conclusion of polymerization was difficult. The Inoue catalysts also generated noticeable amounts of cyclic carbonate by-products and polyether homopolymer that had to be removed from the desired polycarbonate polymer.
Diethylzinc is a highly reactive, pyrophoric material. Stringent handling requirements are necessary to prevent the risk of fire when the material is to be used on a commercial scale. When the polymerization reaction is finished, extensive purification procedures are necessary to remove all traces of catalyst from the polymer. Because of its general lack of stability, sensitivity to moisture and to other catalyst poisons, the active catalyst material is prepared from diethylzinc immediately prior to polymerization. This is a distinct disadvantage since catalyst quality cannot be determined prior to polymerization. These considerations, along with the generally low catalyst productivity, have stimulated the search for improved catalyst materials for epoxide/CO.sub.2 polymerizations.
Zinc carboxylates have also been described as effective catalysts for CO.sub.2 polymerization. Since these are stable materials with none of the handling problems associated with diethylzinc, they represent interesting candidates for a practical commercial catalyst system.
Soga and co-workers reported that reaction products of zinc hydroxide and aliphatic dicarboxylic acids exhibited high activity for the copolymerization of carbon dioxide and propylene oxide (Polymer J. 13(4), 407(1981)). A variety of acids were tested, but only adipic and glutaric acid produced catalysts with higher activity than the known diethylzinc catalysts. Catalysts prepared from aromatic dicarboxylic acids were essentially inert under the Soga polymerization conditions.
Soga reported another approach to improve catalyst activity that involved supporting the catalyst on an inert OXide Carrier (Nippon Kagakkaishi 2, 295(1982)). Using a support increased the surface area of active catalyst material and was expected to lead to more efficient production of polymer. However, the supported catalysts were no more effective than the standard diethylzinc based catalysts.
The metal salts of acetic acid are a third type of catalyst material known to bring about copolymerization of CO.sub.2 with epoxides (Soga. et al., Makromol. Chem. 178, 893(1977)). Only zinc and cobalt produced alternating copolymers from CO.sub.2 and epoxides, and the activity of these catalysts was lower than that observed with diethylzinc derived catalysts.
The synthetic methods described by Inoue, particularly the handling of the organometallic catalyst and its removal from the polymer, proved to be impractical for the production of sufficient polymer for industrial testing and evaluation., therefore, an improved method of polymer preparation was sought. Of all the catalyst systems reported in the literature up to that time, only zinc carboxylates based on adipic or glutaric acid appeared to have the potential for practical use on a commercial scale.
Soga et al reported a synthetic procedure for production of a zinc carboxylate catalyst involving reaction of zinc hydroxide with glutaric acid in alcoholic solvents. The catalysts so obtained experimentally were inconsistent in their polymerization behavior. Some preparations gave products with good productivity while other, seemingly identical, preparations were inactive or had low productivity. In the course of investigating the causes of the inconsistent behavior, an improved method of catalyst preparation was developed. A description of the improved method and the preferred embodiments is described in the next section.
It has also been reported that soluble zinc catalysts can be prepared by the reaction of zinc oxide or zinc salts with a dicarboxylic acid anhydride or monoester in a suitable solvent such as the lower alcohols, ketones, esters and ethers as disclosed by Sun in U.S. Pat. No. 4,783,445.
In the prior art methods, preparation of the catalysts involved repeated washing and purification steps to produce an active catalyst.